NS32FX16-15 Datasheet, PDF (40/88 Pages) National Semiconductor (TI) – Imaging/Signal Processor

3 0 Functional Description (Continued)
TABLE 3-4 Circular Buffer Sizes
DS
Field
External
Buffer
Size (DW)
Constant
Address
bits
Incremented
Address
bits
00
8
A0 A5–A23
A1 – A4
01
16
A0 A6–A23
A1 – A5
10
32
A0 A7–A23
A1 – A6
11
64
A0 A8–A23
A1 – A7
3 5 SYSTEM INTERFACE
This section provides general information on the NS32FX16
interface to the external world Descriptions of the CPU re-
quirements as well as the various bus characteristics are
provided here Details on other device characteristics in-
cluding timing are given in Chapter 4
3 5 1 Power and Grounding
The NS32FX16 requires a single 5V power supply applied
on the V_CC pins These pins should be connected together
by a power (V_CC) plane on the printed circuit board
The grounding connections are made on the GND pins
These pins should be connected together by a ground
(GND) plane on the printed circuit board
Both power and ground connections are shown in Figure
3-14
For optimal noise immunity the power and ground pins
should be connected to V_CC and ground planes respective-
ly If V_CC and ground planes are not used single conductors
should be run directly from each V_CC pin to a power point
and from each GND pin to a ground point Daisy-chained
connections should be avoided
Decoupling capacitors should also be used to keep the
noise level to a minimum Standard 0 1 mF ceramic capaci-
tors can be used for this purpose They should attach to
V_CC GND pins as close as possible to the NS32FX16
During prototype using wire-wrap or similar methods the
capacitors should be soldered directly to the power pins of
the NS32FX16 socket or as close as possible with very
short leads
Design Notes
When constructing a board using high frequency clocks with
multiple lines switching special care should be taken to
avoid resonances on signal lines A separate power and
ground layer is recommended This is true when designing
boards for the NS32FX16 Switching times of under 5 ns on
some lines are possible Resonant frequencies should be
maintained well above the 200 MHz frequency range on
signal paths by keeping traces short and inductance low
Loading capacitance at the end of a transmission line con-
tributes to the resonant frequency and should be minimized
if possible Capacitors should be located as close as possi-
ble across each power and ground pair near the NS32FX16
Power and ground connections are shown in Figure 3-14
3 5 2 Clocking
The NS32FX16 provides an internal oscillator that interacts
with an external clock source through two signals
OSCIN and OSCOUT
TL EE 10818 – 22
FIGURE 3-14 Power and Ground Connections
Either an external single-phase clock signal or a crystal can
be used as the clock source If a single-phase clock source
is used only the connection on OSCIN is required OSC-
OUT should be left unconnected or loaded with no more
than 5 pF of stray capacitance The voltage level require-
ments specified in Section 4 3 must also be met for proper
operation
When operation with a crystal is desired special care
should be taken to minimize stray capacitances and induc-
tances The crystal as well as the external components
should be placed in close proximity to the OSCIN and
OSCOUT pins to keep the printed circuit trace lengths to an
absolute minimum Figure 3-15 and 3-16 show the external
crystal interconnections Table 3-5 provides the crystal
characteristics and the values of the R C and L compo-
nents including stray capacitance required for various fre-
quencies
TL EE 10818 – 23
FIGURE 3-15 Crystal Interconnections 30 MHz
TL EE 10818 – 24
FIGURE 3-16 Crystal Interconnections
40 MHz 50 MHz
40